WordPress applies the permalink change to your entire site. That means that
WordPress updates any posts you already published to the new style. If you’re
switching from the standard style to another style, this never causes a problem,
because the ID-based links continue to work. (That’s because no matter what
permalink style you use, WordPress continues to give each post a unique post
ID, which it stores in its database.) However, if you switch to a second permalink
style (for example, “Day and name”) and then to a third style (say, “Month
and name”), the outlook isn’t so rosy. Anyone who bookmarked a “Day and
name”–style URL will find that it no longer works.
Another important detail is the lifetime of your URLs. Ideally, a good web address
never changes. Think of an address as a contract between you and your readers.
The promise is that if they bookmark a post, the web address will still work when
they return to read it, even months or years later (assuming your entire site hasn’t
gone belly-up in the interim). WordPress takes this principle to heart. In fact, it calls
the unique web address that’s assigned to every post a permalink, emphasizing its
If you want to tweak the way your WordPress site generates permalinks (and if you’re using the
self-hosted version of WordPress, you almost certainly do), it’s best to make that change as soon as possible.
Otherwise, changing the permalink style can break the web addresses for old posts, frustrating readers who have
bookmarked them. Think twice before tampering with the URL structure of an established site.
NOTE WordPress is very conscientious about dealing with old permalinks. If visitors try to find a post using
an old permalink that has since changed, WordPress automatically forwards them to the right post and correct
web address. This trick makes sure that old bookmarks and search engines that link to your site keep working.